The use of radiofrequency (RF) as source of energy for heating underground hydrocarbon-bearing formations is well known. U.S. Pat. Nos. 3,170,519 and 4,620,593 disclose an apparatus to generate the RF at the surface and a coaxial or waveguide to take the energy downhole. U.S. Pat. No. 4,485,868 describes similar equipment with small modifications to be used for electromagnetic heating of hydrocarbon-bearing formations. U.S. Pat. Nos. 4,912,971 and 4,817,711 disclose a downhole microwave generator in which the wellbore is used as a waveguide and the dielectric constants of the formation can be measured and the system can be optimized to reach temperatures up to 400° C. U.S. Pat. Nos. 4,140,180 and 4,485,869 describe three waveguides inserted into the ground to heat a hydrocarbon-bearing formation.
SPE 28619, presented at 69th Annual Tech. Conf. New Orleans, La., USA, Sep. 25-28 (1994) discloses a field test using an RF heating system, including a coaxial line, and a dipole antenna to bring the energy downhole to heat the formation. U.S. Pat. No. 7,891,421 describes a method and apparatus for radiating a RF electromagnetic wave into a hydrocarbon-bearing formation in which two parallel horizontal wells are placed. The RF antenna is configured within the well and allows passage of fluids there through.
Radiofrequency heating has also been disclosed for heating a petroleum/brine-containing formation prior to the injection of any fluid downhole for enhanced oil recovery as in US Pat. App. No. 2014-0262225. Once the formation is heated to a desired temperature, a portion of the indigenous liquids (oil and brine) is produced in order to create a void for the injection of fluids for enhanced oil recovery.
There are many different types of RF antenna that can be used to heat a formation. Some of these antennas can be placed in a well containing nitrogen or other inert gas, while other RF antennas will work better if placed in a well containing an insulating fluid; also known as the “operating fluid”. Allowing an insulating fluid to fill the antenna allows for cooling of hot spots that may develop during operation. This can be accomplished by circulating the insulating fluid through the antenna during operation or by allowing heat transfer by convention and/or conduction. The operating or insulating fluid also serves a role of maintaining pressure balance in the well, thus preventing fluids outside of casing from easily entering the well, which could then short out the antenna.
While RF antennas are known for installation into a wellbore for heating a hydrocarbon-bearing formation, little attention has focused on the real-world issues of operating a high voltage system in the downhole environment, in which brine and other conductive materials from the wellbore, rig, and other equipment as well as metallic fragments remaining in the antenna from construction and installation, may adversely affect antenna performance and operational stability. For example, the fluid within the wellbore will most likely include these “conductive contaminates” because field operations are conducted in a “conductively dirty” environment and the fluid in the wellbore will, unless extreme measures are taken, be “conductively contaminated” with a high enough level of conductive particles. As such, it becomes highly unlikely that a high voltage signal can be applied to the antenna without developing an electrical short.